88 
MESSRS. W. DE LA RUE AND H. W. MULLER OR - THE 
sequently to realise the conjecture that the enormous dilatation which occurred instan¬ 
taneously could have been caused by increase of temperature. On the other hand the 
ascertained facts appear rather to point to the phenomenon being caused by the pro¬ 
jection of the gas-molecules by electrification against the walls of the glass vessel, 
producing thereby effects of pressure, which, however, are distinct from the molecular 
motion induced by heat. 
Recently (July 5th, 1879), Professor Dewar called our attention to a similar phe¬ 
nomenon, which confirms this view. He was working in the chemical laboratory of 
the Royal Institution on the Electric Arc, the source of electricity being a Siemens’ 
dynamo-machine driven by a gas engine. The carbon electrodes were, for the purposes 
of his experiments, enclosed in a metallic cylinder open at the bottom but closed at 
the top ; the bottom was kept air-tight by immersion in mercury. 
The total radiant force was being accurately measured by carrying a stream of water 
through the cylinder into a separate vessel at a known rate, and measuring its increase 
of temperature by very sensitive thermometers, which could be read to a hundredth of 
a degree centigrade ; their rapid pulsations were easily seen as the radiant force varied. 
Suddenly, from time to time, there was an increase of current and a great expansion 
of the volume of the air in the cylinder took place, and, although there were apertures 
in this vessel, the mercury joint was forced by the sudden expansion, and yet no 
sudden rise of the thermometer was noticed when these expansions occurred.'"' 
The arc under various circumstances. 
We now proceed to describe the appearance of the arc with terminals of various 
forms at different distances and with various pressures. It will be seen that the fight 
emitted by different parts of the arc is not of the same intensity throughout, and that 
under most circumstances there is a tendency to break up into distinct entities, 
and ultimately to take a stratified appearance. 
The appearance of the arc, between discs in hydrogen at the various pressures used 
in determining the potential necessary to produce a discharge, is represented in 
Plate 8, figs. 11-19. 
M. 
cells. 
seconds 
11, at 
pressure of 18,684, 
with 
600, from a 
. photograph taken in 50 
12 
,, 58,684 
55 
1200 
55 
„ 50 
13 
„ 141,974 
55 
2400 
55 
„ 50 
14 
„ 252,368 
55 
3600 
55 
„ 50 
15 
,, 386,316 
55 
4800 
55 
„ 20 
16 
„ 558,816 
55 
6300 
55 
„ 4 
17 
„ 558,816 
55 
6300 
55 
„ 1 
18 
,, 651,316 
55 
7760 
55 
55 5 
19 
„ 1,008,421 
55 
10,920 
55 
„ 5 
* De la Rive noticed that oscillations occurred in the.mercury of a gauge attached to an exhausted 
tube as soon as the current passed. In the second and third paragraphs of Note C, in the Appendix, is 
the account of his experiments. 
